Search

In Gasoline Direct Injection (GDI) engines, the secondary break-up plays a significant role in air–fuel mixing. In fact, spray granulometry affects evaporation rate, liquid penetration and plume morphology. Operating pressures and temperatures of both liquid and gaseous phases strongly influence the droplet disruption mechanism in the combustion chamber. In 3D Computational Fluid Dynamics (CFD), several models can be adopted to simulate the secondary break-up process, among which the Reitz-Diwakar and the Kelvin-Helmholtz Rayleigh-Taylor (KHRT) are the most diffused ones in the engine community. However, application of such models in their original versions is limited to a reduced range of injection parameters and ambient conditions. As a matter of fact, large variations of them usually require ad-hoc calibrations of the model constants. To improve the predictive capabilities and reduce the need of case-by-case tuning, an alternative secondary break-up model is proposed in the present paper. It is based on the Reitz-Diwakar one but, compared to the latter, a zonalization of the break-up regimes is proposed. Specifically, it is assumed that only Stripping break-up can occur near the nozzle, while Bag break-up only takes place sufficiently far from it. Moreover, model parameters are now treated as functions of the operating conditions. In particular, the impact of the ambient density on the model parameters is analysed in the present work. The proposed model is calibrated via constant volume vessel simulations on a single-hole GDI research injector at vacuum-to-pressurized conditions (namely at 0.4, 1.0, and 3.0 bar(a) of back pressure), on equal temperature. Model parameters are found to be linear functions of the ambient density. Thereafter, model validation is carried out on two different GDI injectors. The first is again a single-hole (remarkably different compared to the previous one), while the second is a 5-hole prototype. Numerical results provided by the proposed model show a satisfactory agreement compared to the experiments in terms of liquid penetration, Phase Doppler Anemometry (PDA) data and imaging, without any dedicated tuning. Conversely, the Reitz-Diwakar and KHRT models, applied to simulate (with default calibration constants) the same injectors, provide results which remarkably deviate from the experiments.

A novel methodology for the estimation of the mass flow rate delivered by each hole of a multi-hole GDI nozzle is presented and discussed in this paper. The proposed method is based on the measurement of the impact force of each jet and it is able to evaluate the individual hole injection rate and injected mass using the relationship between the spray momentum flux and the mass flow rate. Three different nozzles are tested, one featuring equal hole diameters and two with differentiated hole-to-hole dimensions. Firstly, results are validated in terms of total injection rate, comparing the sum of the individual hole flow rates with the signal from a Zeuch-based flow meter. Secondly, outcomes are compared with the direct measurement of the injected quantity by means of a special device able to collect and weight the fuel delivered from each hole. Results evidence an excellent agreement in terms of mass flow rate dynamic profile as the proposed method is able to detect both opening and closure transients and the static flow rate. The proposed method proved to be able to capture the dynamic mass flow rate in transient conditions, i.e. very short injections, overcoming a limitation of the methodology proposed in a previous study. Moreover, in terms of injected mass, the results show a percentage error lower than 5% for medium and long energizing times and a maximum error of 9.5% for short injections in ballistic operating conditions.

In the optimization of GDI engines, fuel injection plays a crucial role since it can affect the combustion process and, thus, fuel efficiency and pollutant emissions. The challenging task is to obtain the required fuel distribution and atomization inside the combustion chamber over a wide range of engine operating conditions. To achieve such goals, flash-boiling can be exploited. Flash-boiling is a phenomenon occurring when fuel temperature exceeds saturation temperature or, similarly, when ambient pressure is lower than saturation one. Under these conditions, which can occur inside the injector or directly in the combustion chamber, the fuel undergoes extremely accelerated breakup and quickly evaporates. The proposed manuscript shows the application of an alternative flashboiling model, recently implemented by Siemens-PLM in STAR-CD V.2019.1, to be applied in 3D-CFD Lagrangian simulations of GDI sprays. Results are validated against experimental data, provided by the SprayLAB of the University of Perugia, on a single-hole research injector. The new flash-boiling model consists of three main parts: an atomization model able to compute droplet initial conditions and the overall spray cone angle; an evaporation model and, finally, a droplet break-up model; the last two models are designed to simulate all the physical events occurring when droplets are injected into the combustion chamber. As for the investigated operating condition, vessel pressure and temperature are 40 kPa and 293K, respectively; as for the fuel (n-Heptane) temperature, it ranges from 303.15 K to 393.15 K, on equal injection pressure (10 MPa). The numerical-experimental comparison is carried out in terms of liquid penetration, imaging, and droplet sizing.

In direct injection combustion systems, the spray momentum flux measurement can provide significant insight in the fuel jet development and the in-cylinder fuel-air mixing potential. The spray momentum flux can be determined by means of the impact force method, which was proved to be completely consistent when the basic measuring hypotheses are fulfilled, i.e. during the steady phase of the injection process. Conversely, in the transients the measurement technique details can significantly affect the results. An appropriate analysis of the possible effects exerted by the experimental setup is thus mandatory. In order to deepen the knowledge about this measurement technique applied to GDI (Gasoline Direct Injection) systems and to support the design of the experiment, in this paper a CFD 3-D model of a single-hole, research injector was developed and assessed with experimental data in terms of spray penetration curve, overall shape and droplet sizing and velocity. The validated numerical tool was then used to simulate the spray momentum measuring procedure in order to investigate the possible effects of the experimental set-up details on the momentum flux results. To this end, numerical results were compared to experimental data for different values of the main measurement parameters: target size, nozzle/target distance, discharge ambient pressure. This analysis confirmed both the experimental procedure sensitivity to some setup details and the model ability to capture the spray-target interaction phenomenon, supporting the need of a combined use of numerical and experimental approaches to obtain an adequate insight in the spray evolution.

Nowadays, injection rate shaping and multi-pilot events can help to improve fuel efficiency, combustion noise and pollutant emissions in diesel engine, providing high flexibility in the shape of the injection that allows combustion process control. Different strategies can be used in order to obtain the required flexibility in the rate, such as very close pilot injections with almost zero Dwell Time or boot shaped injections with optional pilot injections. Modern Common-Rail Fuel Injection Systems (FIS) should be able to provide these innovative patterns to control the combustion phases intensity for optimal tradeoff between fuel consumption and emission levels.In this work, a 1D-CFD model in GT-SUITE of a solenoid ballistic Common-Rail injector was firstly refined respect to the previous work [1] and then it was validated against an extensive experimental dataset of single injections, standard double pilot and multi-pilot injection patterns (up to 4 pilot events) with almost zero dwell time between two consecutive injection events. The experimental hydraulic test data used to validate the one-dimensional model were obtained by means of the UniPG Injection Analyzer based on the Zeuch's method.The comparison between the experimental and simulated volumetric injection rates showed a more than satisfactory accuracy of the model in predicting the actual behavior of the ballistic injector for all the injection patterns tested, even for relatively complex injector command strategies, characterized by reduced Dwell Time values between consecutive injection events.

Geometric uncertainties in the blade manufacturing process have important consequences in terms of dynamical properties of bladed disks. In this paper, we address the problem of modeling a full bladed disk composed by blades having uncertain geometry. The geometric imperfection of the blades is represented and analyzed according to a procedure previously presented by the authors, based on the principal component analysis (PCA) and the mesh morphing. The dynamical model of the full disk is constructed following the component mode synthesis (CMS) approach. The blade geometry is represented using a probabilistic model constructed from an experimental dataset. The effect of the geometric uncertainties is assessed using a linear uncertainty propagation approach, leading to a procedure that is fast enough to be embedded into a Monte Carlo simulation (MCS) loop.

Fuel injectors featuring differentiated hole-to-hole dimensions improve the fuel distribution in the cylinder ensuring a more efficient and cleaner combustion for GDI (Gasoline Direct Injection) engines. A proper diagnostic system able to detect the actual fuel flow rate exiting each hole of a GDI nozzle is requested in order to optimize the matching between the spray and the combustion chamber.Measuring the spray impact force of a single plume allows the detection of the momentum flux exiting the single hole and, under appropriate hypotheses, the evaluation of the corresponding mass flow rate time-profile. In this paper two methodologies for the hole-specific flow rate evaluation, both based on the spray momentum technique, were applied to two different GDI nozzles, one featuring equal hole dimensions and one with two larger holes. Three different energizing times at 100 bar of fuel pressure were tested in order to cover a wide range of operating conditions.The results were validated in terms of injected mass by means of a proper device able to collect and weigh the fuel injected by each single nozzle hole, and in terms of mass flow rate using a Zeuch-method flow meter as reference.Both the proposed methodologies showed an excellent accuracy in the fuel amount detection with percentage error lower than 5% for standard energizing times and lower than 10% for very short injections working in ballistic conditions.The mass flow rate time-profile proved a good accuracy in the detection of the start and end of injection and the static flow rate level.Fuel injectors featuring differentiated hole-to-hole dimensions improve the fuel distribution in the cylinder ensuring a more efficient and cleaner combustion for GDI (Gasoline Direct Injection) engines. A proper diagnostic system able to detect the actual fuel flow rate exiting each hole of a GDI nozzle is requested in order to optimize the matching between the spray and the combustion chamber.Measuring the spray impact force of a single plume allows the detection of the momentum flux exiting the single hole and, under appropriate hypotheses, the evaluation of the corresponding mass flow rate time-profile. In this paper two methodologies for the hole-specific flow rate evaluation, both based on the spray momentum technique, were applied to two different GDI nozzles, one featuring equal hole dimensions and one with two larger holes. Three different energizing times at 100 bar of fuel pressure were tested in order to cover a wide range of operating conditions.The results were validated in terms of injected...

The scientific literature focusing on the numerical simulation of fuel sprays is plenty of atomization and secondary break-up models, all aiming at simulating GDI sprays under possibly any engine condition in terms of injection pressure and cylinder back pressure. However, it is well known that the predictive capabilities of even the most diffused break-up models are affected by injection parameters, especially backpressure. As a consequence, model constants require usually substantial tuning based on the specific operating conditions. In this manuscript, an alternative atomization methodology is proposed for the 3D-CFD simulation of GDI sprays, aiming at reducing case-to-case tuning of the model constants for variations of the operating conditions. In particular, attention is focused on the effects of back pressure, which has a huge impact on both the morphology and the sizing of GDI sprays. 3D-CFD Lagrangian simulations of two different multi-hole GDI injectors are presented. The first injector is a 5-hole GDI prototype unit operated at ambient conditions. The second one is the well-known Spray G, characterized by a higher back pressure (up to 0.6 MPa). Results are compared against experimental data in terms of liquid penetration, PDA (Phase Doppler Anemometry) data of droplet sizing / velocity and imaging. CFD results are demonstrated to be highly sensitive to the spray vessel pressure, mainly because of the atomization strategy. The proposed alternative approach proves to strongly reduce such dependency. To confirm the improvements, such approach is combined to two different well-known secondary break-up models, namely Reitz’s model and the KHRT one.

The increase of injection pressure in GDI engines represents a proven development trend in the effort of limiting raw particulate emission, which is one of the most severe concerns for this kind of combustion system. Since increasing injection pressure level tends to cause significant issues in terms of spray penetration control, in parallel with ultra-high injection pressure multi-event strategies are being considered to improve the air-fuel mixture formation management. The adoption of these advanced injection strategies can potentially affect the injection system hydraulic behavior, hence an adequate control of each event fuel quantity is mandatory for a proper engine optimization. When the injector is operated according to a multi-event strategy, complex hydraulic and electro-magnetic phenomena may occur. Pressure waves triggered by each injection can eventually influence the subsequent events, modifying the injected volume compared to the expected, isolated event value. This hydraulic phenomenon is well-known for Diesel applications and is investigated in the present paper in a typical GDI configuration evidencing the effect of the dwell time value between consecutive events both in terms of injected volume and injection rate profile using a proprietary injection analyzer. Further, when consecutive injections are remarkably close, the succeeding injection event is likely to be influenced by a different, electro-magnetic phenomenon in addition to pressure waves propagation in the system pipes. With reduced dwell time strategies, the current profile driving the injector can be modified by a previous close-coupled actuation due to residual magnetization in the solenoid circuit. This phenomenon determines for the succeeding injection event a steeper transient of the solenoidal current, causing a faster injector needle rise and correspondingly a significantly increased injected quantity. In this paper, an innovative methodology is proposed to investigate in multi-injection strategies the effects of close injector actuations, evidencing the influence of hydraulic phenomena and of current profile modifications, respectively. The developed approach, named ETboost, consists in a double injection with a first dummy command. To this end, a short ETboost command is required not causing the injector needle opening and thus no triggering pressure waves that might influence the second injection. Using this analysis approach, the residual magnetization effects on a production GDI injector were experimentally investigated in terms of mean injected volume and instantaneous flow rate. The results obtained with reduced dwell-time were compared with the standard operation mode (single injection), evidencing a significant increment of the injected volume in particular in the ballistic operation where the actual injected quantity can be more than doubled if compared to standard actuation. The dependence of the injected volume on ETboost and dwell time was explored for different operating conditions, evidencing how reducing the dwell time and increasing the ETboost duration the injected volume is enlarged and the shot-to-shot dispersion is significantly reduced.

The scientific literature focusing on the numerical simulation of fuel sprays is rich in atomization and secondary break-up models. However, it is well known that the predictive capability of even the most diffused models is affected by the combination of injection parameters and operating conditions, especially backpressure. In this paper, an alternative atomization strategy is proposed for the 3D-Computational Fluid Dynamics (CFD) simulation of Gasoline Direct Injection (GDI) sprays, aiming at extending simulation predictive capabilities over a wider range of operating conditions. In particular, attention is focused on the effects of back pressure, which has a remarkable impact on both the morphology and the sizing of GDI sprays. 3D-CFD Lagrangian simulations of two different multi-hole injectors are presented. The first injector is a 5-hole GDI prototype unit operated at ambient conditions. The second one is the well-known Spray G, characterized by a higher back pressure (up to 0.6 MPa). Numerical results are compared against experiments in terms of liquid penetration and Phase Doppler Anemometry (PDA) data of droplet sizing/velocity and imaging. CFD results are demonstrated to be highly sensitive to spray vessel pressure, mainly because of the atomization strategy. The proposed alternative approach proves to strongly reduce such dependency. Moreover, in order to further validate the alternative primary break-up strategy adopted for the initialization of the droplets, an internal nozzle flow simulation is carried out on the Spray G injector, able to provide information on the characteristic diameter of the liquid column exiting from the nozzle.

Achieving an adequate knowledge of the fuel injector dynamic operation is an important step in the global effort of improving direct injection gasoline engine efficiency while limiting pollutant emissions. In this frame, needle displacement measurement is an important add-on to evaluate the injector actual performances and can be very useful for developing predictive numerical models to support both the component and engine development. In this study the dynamic behavior of a GDI injector was investigated by the simultaneous measurement of injector needle lift and of the resulting injection rate. With this methodology, the needle displacement is measured by laser Doppler vibrometry, while the injector is tested by an injection analyzer based on the Zeuch method. The vibrometer laser beam points on the needle back surface, passing through a quartz window allowing the fuel pressurization and the normal injector operation. The analysis of the simultaneously acquired needle displacement, injection rate, fuel inlet pressure and current time-history profiles allows a complete insight in the injector dynamic operation. The pressure effect on the needle dynamics was assessed with a detailed analysis of the opening and closing transients; further, multiple actuation strategies were analyzed focusing on the influence of dwell time on the injector dynamics evidencing how the resulting injection rate profile is affected by the needle inertia and by the pressure wave propagation along the fuel line.

The component mode synthesis based on the Craig-Bampton method has two strong limitations that appear when the number of the interface degrees of freedom is large. First, the reduced-order model obtained is overweighed by many unnecessary degrees of freedom. Second, the reduction step may become extremely time consuming. Several interface reduction techniques addressed successfully the former problem, while the latter remains open. In this paper we tackle this latter problem through a simple interface-reduction technique based on an a-priory choice of the interface modes. An efficient representation of the interface displacement field is achieved adopting a set of orthogonal basis functions determined by the interface geometry.

Geometric uncertainties involved in the rotor blade manufacturing process are a major concern for designers. The deviation of the produced components from their nominal geometry have an impact on the Natural Frequencies (NF) that, under certain circumstances, may have negative effects on the dynamic forced response in operative conditions. Geometric defects are usually limited by imposing dimensional tolerances based on empirical considerations, simplified approach that may lead to costly manufacturing requirements that still may not guarantee safe results. This paper proposes a probabilistic representation of the geometric uncertainties for rotor blades and defines a procedure to evaluate their effects on the blade NFs. The deviation from nominal geometry is represented through the Principal Component Analysis (PCA) where it is expressed as a sum of characteristic geometric shapes (GUMs) modulated by mutually uncorrelated random variables (Principal Components, PC). The effect of each GUM is then linearly propagated on the blade NFs and a sensitivity matrix is finally defined. The procedure is applied to a case-study that concerns a set of 50 nominally identical compressor blades and the ability of GUMs to represent the effects of geometric uncertainties is tested.

A N-heptane spray from a GDI multi-hole injector operated in ambient air at fixed conditions and with doubleinjection commands is studied with different experimental techniques to better understand the spray behaviors, focusing the analysis on the effect of different dwell times between the two pulses. Results from spray photographic analysis, fuel injected quantity, droplet velocity and sizing by Phase Doppler Anemometry are presented and compared. The peculiarities and usefulness of a complementary application of the different techniques is illustrated. The two spray pulses have the same time length, so that the first spray evolves in a nearly quiescent and clean ambient, while the second, nominally identical to the first one, evolves in its trailing edge. The direct comparison allows an immediate perception of the differences among the two sprays, at the different dwell times, where the shorter tested, 160 microseconds, was chosen as the one that shows the first appreciable effect with at least one of the used techniques; the differences are clearly evident in the PDA results, sufficiently visible from the injection rate, not appreciable in the imaging at short distance. The effect of the longerdwell times becomes more evident and is illustrated.DOI: http://dx.doi.org/10.4995/ILASS2017.2017.5007

The influence of the bond pattern on the in-plane limit strength of masonry is analyzed through a simplified procedure based on the application of the safe theorem of limit analysis to the unit cell that generates the whole masonry by periodic repetition. The limit strength domains of running bond, English bond and herringbone bond masonry are obtained with different orientations of the mortar bed joints with respect to the principal directions of the average stress. The effects of different brick geometries are analyzed and comparisons between strength properties of different masonry patterns are made.

Conventional injection is still the leading method to deliver macromolecular therapeutics. Needle injection is considered a low compliance administration strategy, principally due to pain and needle phobia. This has fostered the research on the development of alternative strategies to circumvent the skin barrier. Among needle-free drug delivery methods, jet injection is an old strategy with great potential not yet completely disclosed. Here, the design, engineering and dynamic behavior of a novel spring-powered micronozzle needle-free injector is presented. Fluid mechanics was first studied in air to calculate jet force and speed as well as injection duration in different conditions. Polyacrylamide gel was used to simulate a soft tissue and to investigate the jet evolution over time of different injected doses. Finally, ex vivo characterization was carried out on pig skin. Results evidenced a direct dependence of the force, velocity, and duration with the injection volume. The model material allowed individuating the different steps of jet penetration and to attempt a mechanistic explanation. A different behavior has been recorded in the skin with interesting findings for subcutaneous and/or dermal delivery. Peculiar features with respect to existing jet injectors confers to this device good potentiality for a future clinical application. (C) 2015 Elsevier B. V. All rights reserved.

A two-dimensional model for the evaluation of the load carrying capacity of masonry bridges is proposed that takes into account the strengthening effects due to arch-fill interaction observed in experimental tests. Upper bounds on the collapse load and the corresponding mechanism are obtained by means of a finite element application of the Kinematic Theorem of Limit Analysis. Arches and piers are modelled as beams made up of non-tensile resistant (NTR) and ductile in compression masonry, while the fill is represented as a cohesive-frictional material with tension cut-off. Kinematically admissible mechanisms are obtained by discretizing the fill domain with triangular elements and including velocity discontinuities in order to increase the degrees of freedom of the model and thus reduce locking; arches and piers are discretized by two-node straight beam elements. A piecewise linearization of the limit domains allows the upper bound on the collapse load and the corresponding mechanism to be obtained as a solution of a Linear Programming problem. The capabilities and the validity limits of the proposed numerical model are shown in two examples. The collapse test on a real single span bridge is simulated and discussed in the first example; a multi-span bridge is analysed in the second, where complex interactions between arches, piers and fill are obtained.

Experimental collapse tests on full and model scale masonry bridges have shown that fill and spandrels can strongly affect the collapse behaviour and increase the load carrying capacity. To provide a structural description of the arch–fill interaction effects, a two-dimensional model of multi-span masonry bridges is developed in which arches and piers are described as beams made up of elastic, non-tensile resistant (NTR), ductile in compression material, and the fill as a Mohr–Coulomb material modified by a tension cut-off under plane strain conditions. The load carrying capacity is evaluated by a finite element limit analysis procedure based on the kinematic theorem. The fill domain is discretized by triangular elements connected by interface elements in order to allow possible velocity discontinuities at common sides of adjacent triangular elements; arches and piers are discretized by two-noded straight beam elements. By linearization of the limit domains in the generalized stress space, a linear programming problem is formulated and upper bounds of the collapse loads are obtained. Two examples are discussed, concerning a real single-span bridge, tested up to collapse, and a multi-span bridge. The ideal ductility assumption implicit in limit analysis is discussed by comparing the upper bound evaluations to the results obtained by incremental analysis in order to obtain the validity limits of the upper bound limit analysis for the proposed model.

The interaction of multiple delaminations in a laminated composite plate loaded dynamically under plane strain conditions (cylindrical bending) is studied by a simple but accurate model that represents the delaminated plate as a set of Timoshenko beams joined by cohesive interfaces. Behavioral maps are derived, which distinguish conditions under which multiple delaminations tend to propagate with equal lengths from those under which one of them tends to grow as a dominant crack with relatively high velocity. In homogeneous systems, equal length growth is favored when the delaminations are equally spaced through the thickness. While the behavioral maps are similar to those for static loading conditions, significant dynamic effects arise in the details of propagation: the maximum energy release rate depends strongly on the loading rate, duration and profile; dynamic effects and crack-interaction effects are generally coupled; and strong hammering effects (chaotic collisions of sub-laminates) can occur during the free wave motions that arise after the load is removed. The hammering effect can be suppressed by imposing a large-scale bridging mechanism (bridging extending far in the crack wake, as from pins or stitches), whereupon energy release rates tend to show smooth oscillations associated with waves propagating on the scale of the whole specimen. The energy absorbed during failure will depend significantly on whether conditions favor multiple delaminations propagating with equal lengths or a single delamination growing dominantly.